Backscatter sensors are used in a wide range of technological fields, including sensor applications for sensing a range of parameters such as temperature, pressure, torque, mass, humidity, and chemical vapors. Such backscatter sensors have been realized using LC-resonator, surface acoustic wave (SAW) or bulk acoustic wave (BAW) technology. In considering factors such as cost and effectiveness, systems that use the ultra-high frequencies (UHF) in the industrial, scientific and medical (ISM) radio bands are generally of most interest. The main advantages offered by a backscatter sensor are the combined wireless and battery-less operation.
However, known devices employing the above-mentioned technologies have various associated disadvantages. Known backscatter sensing systems normally suffer from large losses owing to, for example, low Q-factor (large damping).
LC-resonators are only useful for very low frequencies (typically <1 MHz) because of a degradation in the Q-factor of such devices at higher frequencies.
In principle, a backscatter sensor can be realized as either a delay line system or a resonator circuit, and each of these methods can be realized using a method of direct perturbation (i.e., where the measurand directly influences the BAW- or SAW-element) or one of indirect perturbation (i.e., where the measurand directly influences a circuit element, such as a capacitor, which in turn is coupled in a circuit to the BAW- or SAW-element). For a delay line configuration the interrogation (or irradiation) signal is normally a pulse, while for the resonator circuit the interrogation (or irradiation) signal is normally a modulated (AM or FM) continuous wave. In practice, the SAW configuration can be used for both direct and indirect perturbation. However, the BAW configuration is generally only suitable for use with indirect perturbation. In the case of direct perturbation the sensitivity will typically be proportional to the amount of energy in the propagation (acoustic) path that is perturbed, making BAW sensors less sensitive than SAW sensors as the energy is dispersed through the bulk material, minimizing the energy density on the surface where sensing occurs.
A typical example of a BAW backscatter sensor is known to be used as a sensor in a vehicle tire that is irradiated by an antenna in the wheel arch at a carrier frequency of 2.45 GHz (maximum power of 10 mW). Initially, the signal is amplitude-modulated by a control unit in the range of 5 to 10 MHz. The sensor receives the signal and demodulates it by means of a detector diode; the modulated wave is used to stimulate oscillations in a quartz crystal resonator. The modulation is then switched off, and the carrier signal is radiated at reduced power. The quartz then vibrates at its natural resonance frequency, which varies with temperature or as it is influenced by an associated capacitive pressure sensor. These vibrations are mixed with the remaining carrier signal, which is then reflected to the antenna, this reflected signal including modulated sidebands. The control unit receives the signal and analyzes it by means of a digital receiver circuit.
A typical example of a known BAW backscatter sensor is shown in FIG. 1a. The configuration includes a varactor diode 1, a quartz resonator 2 and a capacitive sensor 3. This system is disadvantageous owing to the requirement for each of the varactor diode, quartz crystal and capacitive pressure sensor, creating unnecessarily complex circuitry. Hybrid integration is necessary, while such a sensor is relatively large, heavy and expensive to manufacture. The Q-factor of such a device is limited by the series resistance in the capacitive pressure sensor (or impedance sensor). Additionally, such a device is only operable within a limited range of frequencies.
Typical examples of a SAW backscatter sensor are also known as Acoustic Wave Technology Sensors. These are disadvantageous in that they rely on customized technology and are mechanically complex. They have a very high relative size and weight compared to other types of backscatter sensors, are expensive to manufacture and again have a limited Q-factor.
A known force sensor has a beam of silicon material that is subjected to vibration at a resonant frequency, the vibration frequency changing due to applied forces acting on the sensor.
There is therefore a need within the field of backscatter sensors to provide a simplified and inexpensive sensor that is compact and light compared to current devices, that has a less complex mechanical structure and that provides high Q-factors and that can accommodate improved detection over a wide range of frequencies.